Evidence for accelerated mouse sperm penetration into eggs in vitro, and hence for accelerated capacitation, was obtained when the fertilizing ability of sperm suspensions was tested after preincubation, for a period of time insufficient to permit full capacitation under standard conditions, in the presence or absence of caffeine. When eggs were fixed after 1 h, the majority (76.7%) in the caffeine-containing medium were fertilized compared with only 32.1% in the caffeine-free medium. When fixation was later, after 1 h 15 min, fertilization levels were high (approximately 90%) in both groups, but significant differences in the stages of egg activation and sperm head decondensation reached were observed. In the absence of caffeine, early stages of nuclear development were predominant, while in the presence of caffeine, a significant proportion of eggs had reached terminal stages of activation. There was also some evidence for precocious sperm, but not egg, nuclear development when fertilization occurred in caffeine-containing medium.